Portable electronic devices often include batteries to power the devices when power outlets may not be available, convenient, or functioning. As portable electronic devices have become more powerful (for example, by inclusion of more features), the batteries used in conjunction with them have tended to grow in capacity as measured by their ampere-hour ratings. Many users of these electronic devices prefer lithium-ion batteries because of their high storage capacity. Moreover, lithium-ion batteries can be re-charged, have high capacity-to-weight ratios, and retain their charges well when not powering the device. Thus, many users power their portable electronic devices (such as laptop computers, telecommunication equipment, entertainment devices, etc.) with lithium-ion batteries.
Batteries in general, and more particularly lithium-ion batteries, have certain disadvantages. For instance, some lithium-ion batteries tend to warm during operation due to Joule heating within the batteries. Joule heating arises from the battery generated current flowing through internal features of the battery which offer some resistance to the flow of that current. Joule heating increases in proportion to the square of the current. Thus, when large current draws occur on the battery, Joule heating can increase sharply thereby driving the battery temperature higher. As the temperature of some lithium-ion batteries increases, cells within the batteries can become unstable and begin internally discharging across their negative and positive terminals. This discharge can generate Joule heating and warm the battery further. In turn, the increasing temperature causes further instability, discharge, and (potentially) the loss of the battery.
Another disadvantage associated with using batteries to power portable electronic devices arises from various mechanical features of the batteries. More particularly, lithium-ion batteries typically include numerous cells each of which has a layer of carbon and a layer of lithium cobalt oxide separated by a separator. The carbon layer is typically connected to the negative terminal whereas the lithium cobalt layer is typically connected to the positive terminal of the cell. The separator is usually a sheet of insulating material. Should the case of a lithium-ion cell be punctured the separator can be damaged thereby creating an electrical “short” circuit between the electrodes. Short-circuited cells can discharge across their electrodes leading to rapid warming of the cell, possible loss of the cell, and warming of adjacent cells with an attendant possibility that these adjacent cells might also be lost. Yet, as more electronic devices become portable and require increasing amounts of power, the possibility that one or more cells might be subject to mechanical abuse increases.
In addition, high storage and operating temperatures can disadvantageously affect batteries too. For instance, battery life tends to decrease with increasing operating and storage temperatures. Indeed, lithium-ion batteries (some of which loose capacity with time regardless of their thermal environment) can lose capacity at an accelerated rate when stored or operated in warm environments. Yet, as more electronic devices become portable, their batteries are becoming increasingly exposed to wide thermal variations.